The myelin-associated glycoprotein (MAG) is a member of the immunoglobulin gene superfamily that is localized in the periaxonal membranes of PNS and CNS myelin sheaths where it functions in glia~axon interactions. It occurs in two developmentally regulated isoforms with differing C~terminal tails generated by alternative splicing of its mRNA. The carbohydrate in MAG consists of a mixture of oligosaccharides, many of which are sialylated and sulfated, and which are currently being isolated and characterized. During this year, it was demonstrated that the larger apparent Mr of MAG in the dysmyelinating quaking mouse is due to greater glycosylation in the mutant, especially sialylation. The expression of MAG in cultured oligodendrocytes and Schwann cells is being studied with the ultimate objectives of identifying factors that control its synthesis and probing its function in cell~cell interactions. Although cultured primary Schwann cells do not normally express MAG in the absence of neurons, some immortalized Schwann cell lines generated in our laboratory express remarkably high levels of MAG. It has now been demonstrated that the amount of MAG expressed by these lines is greater when their rate of growth is reduced by culturing in defined media or when the cells reach high density. These findings suggest that the level of MAG expression is inversely related to the rate of cell division. MAG is phosphorylated both in the Schwann cell lines and in cultured oligodendrocytes, and in both cases the phosphorylation is catalyzed at least in part by protein kinase C and calcium~activated kinases. However, phosphorylation in the two cell types differs in that it is almost exclusively on serine residues of the small MAG isoform in Schwann cells, whereas it is on both isoforms and on serine, threonine and tyrosine residues in oligodendrocytes. Addition of exogenous GM3 ganglioside to the culture media of oligodendrocytes stimulates the formation of processes and promotes differentiation in the direction of myelination as indicated by increased synthesis of galactocerebroside, sulfatide and MAG. The treatment also stimulates the phosphorylation of MAG, although there is generally a down~regulation of phosphorylation of most proteins in the treated cells. Since cultured oligodendrocytes normally synthesize increasing amounts of GM3 as they differentiate in culture, it seems likely that GM3 is an essential component of their surface membrane that is needed in the preparation to myelinate.